According to the source material, the pressure inside a spherical soap bubble ($P_i$) is greater than the atmospheric pressure outside ($P_o$) due to surface tension . A soap bubble has two liquid-air interfaces, and the excess pressure is calculated using the formula $P_i - P_o = 4S/r$, where $S$ represents the surface tension and $r$ is the radius of the bubble . From this relation, the total pressure inside is $P_i = P_o + 4S/r$. If the bubble expands, its radius ($r$) increases. Since the internal pressure is inversely proportional to the radius, an increase in $r$ causes the excess pressure term ($4S/r$) to become smaller, thus the overall pressure inside the bubble decreases.